There is evidence that root surface caries, like coronal caries, result from the metabolic activities of plaque microorganisms, although it is not certain yet which are the major pathogenic bacteria. Actinomyces and streptococci have been detected in root surface plaque and have been implicated in the carious process. These microorganisms can accumulate and be affected by fluoride. It is the aim of the proposed study to examine the basic aspects of the interactions of root surface plaque with fluoride. Fluoride will be measured in root surface plaque and the observed concentrations will be compared to those existing in coronal plaque. Further comparisons will be made between the plaque F content and the bacterial composition of the plaque. In addition, the effects of waterborne F and topical F preparations as well as other factors on the concentrations of F in plaque will be examined. In previous studies we have shown that the acidogenic activities of the Actinomyces are less affected by F than are those of the streptococci. Furthermore, we have found that different strains of the streptococci exhibit significantly different sensitivities to low levels of the agent. It is proposed, therefore, that the sensitivities of plaque samples to F will reflect the nature of the indigenous microorganisms. The metabolic sensitivies of samples of intact plaque to low and high concentrations of F will be evaluated and the assay systems will be applied to the comparison of root surface and coronal plaque, and to the evaluation of plaque from patients with different caries experience, and health status. The results that will be obtained from these studies will provide information about the basic nature of the interactions between F and root surface plaque. In addition, the proposed experiments may enable us to better evaluate the therapeutic potential of topical F, and may lead to new strategies for the use of F in the preventive treatment of exposed root surfaces.